Genetics Exam 2

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alleles

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alleles

Alternate forms of the same gene

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Wild type

-The allele that occurs most commonly in a population -May be dominant or recessive

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mutant allele

-allele caused by a mutation in the wild-type allele -May specify an alternate gene product or inactivate the gene

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null allele

•A gene that does not produce a functional gene product due to a mutation in the wild-type gene •Usually recessive

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when the mutant allele is dominant

when naming alleles for model organisms, when is upper case used?

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complete dominance

The heterozygote is phenotypically indistinguishable from the homozygote

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incomplete dominance

Phenotype is intermediate between the two homozygous phenotypes

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heterozygote advantage

-gives carriers with one affected allele benefit in another area -Helps explain why some mutations are maintained in a population

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Pleiotropy

A single gene can present multiple effects

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Codominance

Heterozygote shows the phenotypic effects of both alleles equally

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polymorphic

name for locus with multiple functional alleles

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lethal alleles

•Mutations in essential genes •May be dominant or recessive

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recessive lethal alleles

give 2:1 phenotypic ratio in monohybrid crosses

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gene interactions

The situation in which more than one gene is involved in determining a particular phenotype

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epistasis

the effect of one gene pair masks or modifies the effect of another gene pair

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epistatic

•name for the gene that masks or modifies •may be dominant or recessive

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hypostatic

The gene whose effects are masked or modified

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recessive epistasis

-Two recessive alleles at the epistatic locus prevent expression of the alleles at the hypostatic locus -results in modified phenotypic ratio

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dominant epistasis

•A dominant allele at the epistatic locus prevents expression of the alleles at the hypostatic locus

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complementation

The presence of at least one dominant allele at each locus is necessary for wild-type phenotype

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Homogametic sex

Produces gametes containing only one kind of sex chromosome

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Heterogametic sex

Produces gametes which carry one of two different kinds of sex chromosomes

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heteromorphic

chromosomes with different in size and/or centromere position

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Thomas Hunt Morgan

•Demonstrated that gene was located on the X chromosome •First data in support of the chromosome theory of heredity

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sex linkage

Genes located on the sex chromosomes show different patterns of inheritance than autosomal genes

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male

in x-linked recessive disorders which sex shows the trait more

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a recessive trait is not sex linked

what does a mating involving an affected female in which some or all sons are not affected indicates

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x linked dominant disorders

-Very rare -Affected males pass on condition to all of their daughters, but none of their sons

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a dominant trait is not x linked

what does a mating involving an affected male in which some or all daughters are not affected indicate?

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sex-limited heredity

•Phenotype is autosomally transmitted •Expressed in only one sex

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Sex-influenced heredity

•Autosomally transmitted •Displayed in both males and females •Expression is dependent upon hormone constitution, so expression patters differ between males and females

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variable expressivity

Individuals with the same genotype differ with respect to the severity of the phenotype

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penetrance

The probability that an individual who has the genotype associated with the trait will display the phenotype, affected interactions with the internal cellular environment and the external environment

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methylation

how can genes be turned off in genomic imprinting

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discontinuous traits

•characteristics with a small number of discrete phenotypes •Usually determined by one or a few gene loci

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multifactorial

Having both genetic and environmental components

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Bateson and Yule

who proposed the multiple factor/multiple gene hypothesis

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Hermann Nilsson-Ehle

-performed experiments on grain color in wheat that were the basis for multiple factor hypothesis -proposed additive and nonadditive alleles

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additive allele

Contributes a certain amount to the phenotype

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nonadditive alleles

Does not contribute to the phenotype

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Multiple-factor hypothesis

-Traits are polygenic -Many genes, each behaving in a Mendelian fashion, contribute towards the phenotype -Each gene locus may be occupied by either an additive or nonadditive allele -Together, the additive alleles produce substantial phenotypic variation

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equal

In the multiple factor hypothesis, the contribution of each additive allele to the final phenotype is approximately _____

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continuous traits

Phenotypic variation can lie at any point within a set range of measurements

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meristic traits

Traits in which the phenotype can be expressed as a whole number

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Threshold Traits

-Typically have a small number of distinct phenotypic classes -The combination of alleles at multiple loci gives an individual a greater or lesser likelihood of showing one of these phenotypes

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complementation analysis

used to determine whether two independently isolated mutations are due to mutations in the same or different genes

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the mutations are in different genes

In complementation analysis 2 isolated mutants are crossed, what does wild type progeny indicate?

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Bateson and Punnett

Who published the first study of linked genes about sweet pea plants inn 1906?

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coupling between dominant and recessive alleles

What did bateson and punnett propose?

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Thomas Hunt Morgan

Who hypothesized that parental (non-recombinant) types are more common than non-parental?

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They are physically linked

What did Hunt Morgan propose about genes inherited together?

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FA Janssens

Who first described chiasmata as the sites of crossing over in 1909?

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50%

What is the theoretical limitation to recombination with crossing over?

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no

does crossing over always occur between genes?

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recombinant frequency

An estimate of the proportion of chromosomes in which a crossover occurred between genes that is proportional to the distance between two linked genes

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interchromosomal recombination

-recombination due to independent assortment -R.F. = 50%

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Intrachromosomal recombination

-recombination due to crossing over

  • R.F. < 50%

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linkage

The situation where gene pairs reside on the same chromosome

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Linkage group

A group of genes having their loci on the same chromosome

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the number of chromosomes

Number of linkage groups should be the same as what?

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linkage phase

Combination of alleles linked on the same chromosome

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coupling conformation

AB or ab

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repulsion conformation

Ab or aB

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complete

if only paternal chromosome types are produced linkage is _____

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Alfred Sturtevant

Used test cross data to map the relative distance between linked genes

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increases

The frequency of crossing over ______ as the distance between two genes increases

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three point testcross

Aids in detection of double crossovers (DCOs) and allows relative order of genes on a chromosomes to be established with one cross

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AaBbCc x aabbcc

what is crossed in an autosomal 3 point test cross?

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AaBbCc female and abc/Y male

what is crossed in a sex-linked 3 point test cross?

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NCOs

What progeny type are most common?

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DCOs

What progeny type are least common?

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in the middle

Where is the gene that is out of phase in the DCOs and NCOs located?

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Add the 2 shorter distances together or add 2x the dcos to the formula.

How do you find the RF for the two most distant genes?

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Microsatellites (STRs)

• Short sequences (1 to ~6 bp long) • Tandemly repeated • Found throughout the genome • E.g., CACACACA

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in length

how do heterozygote microsatellite alelles differ?

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Expansion repeat disorder

Disease occuring when the number of copies of a normally occurring STR increases to a critical number

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minisatellites

Similar to microsatellites • Tandemly repeated sequence • Number of repeats varies, constituting different alleles • Repeat sequence is longer (10 to 100 nucleotides)

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Single Nucleotide Polymorphisms

Variations at a single base pair

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80

SNPs comprise over ____% of the 0.1% variation in the human genome

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XX/XY

What kind of sex determination do humans use?

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protenator mode

XX/X0 mode of chromosomal sex determination

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lygaeus mode

XX/XY mode of chromosomal sex determination

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heterochromatic

A large portion of the Y chromosome is _______

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Pseudoautosomal regions

-On both ends of the Y chromosome -Share homology with regions on both ends of the X chromosome

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